less hum: switching power supply?

[Roel Das]

Hello,

In the 78xx datasheet, there's a schematic given for high input voltage
regulators. They just put a resistor on the input of the regulator (before
the cap!). Isn't 35V quite high to feed to a 7815/7915? Wouldn't it be
'cooler' if you put it there?

What is the preferred input for for example a 7815?

The protection diodes are also covered in the datasheet. And a shortcircuit
protection using 2 transistors; is this worth the trouble? Anyone of you
ever got a shortcircuit from normal use of equipement? (Not development,
just use...)

And I thought I would know it all by the end of the year...

Roel

>
>     You are correct Paul.
>
>     The biggest cause of failure I have noted in both 78xx and 79xx
regulators
> has been missuse by the designer.  Probably the single biggest crime is
the
> overheating protection.  While the regulators will shut down when they get
too
> hot, I have seen many engineers use this as an excuse to inadequately
heatsink
> the regulators.  Overheating protection or not, one rule of thumb I
remember
> (perhaps a little dimmly) is that for every 10 degree C rise in temp, you
double
> the failure rate (this is for semiconductors, don't really know how true
this
> rule of thumb is).  78xx/79xx parts basically should not fail.  If you are
> seeing a significant number of failures, something is wrong.
>
>     Also, for the price of two more diodes, you can protect the regulators
> against reverse voltage damage quite easily (as per National Semiconductor
> Application Notes).  If you wish to see how to connect these diodes you
can look
> at http://www.silcom.com/~patchell/powersupply/powersupply.pdf for an
example.
>
>     -Jim
>
> >
> > perhaps this is time to note that not all chips with the same type #
> > are always identical.
> >
> > paul perry Melbourne Australia
>
>